Thanks for the input. If your ped used to run ok, I would wonder if the main jet and/or the needle jet (or it"s equivalent), are corroded. I have had problems with old motorcycle jets which had a dull or etched surface causing a rich mixture. This was only visible to me under a jeweler's loupe. Replacement with shiny jets solved the problem.
I would not be suspicios of the float needle and seat if the plug does not foul at idle, when even a slight richness causes great problems. If your carb has a brass float, make sure it has nothing inside it, if it is composite, it may have absorbed fuel making your float bowl level too high. Did you poke anything through the jet to get the sludge out of it?
Having two of the same model moped, you do have the option of swapping carbs, and/or parts, provided that you check the jet #'s etc. Also, make sure that the air cleaner element is clean and dry.
On my over rich ped, even the air cleaner housing without the element was enough of a restriction to have a noticeable effect.
My understanding of the four/cycling symptom, without going back to my 2 cycle book and re-reading it, is that a rich mixture burns more slowly sothat the incoming charge has to fight a still burning mixture. This results in a non combustible mixture for that cycle. On the next cycle, the combustion chamber is flushed outand the mixture is ok.